This invention relates generally to microactuators. More particularly, it relates to two-dimensional gimbaled scanning actuators with vertical comb-drives for actuation and/or sensing.
Microelectromechanical system (MEMS) fabricated using silicon integrated circuit processing techniques have been developed for a wide variety of applications that require actuation and/or sensing of microstructures. The electrostatic comb-drive structure has become an integrated component in many of these MEMS device. A vertical comb-drive device can be used to generate an actuating force on a suspended structure as a bias voltage is applied. This force can be used to actuate microstructures out of the plane in which they were made.
Electrostatically actuated gimbaled two-dimensional actuators have previously employed an electrostatic gap-actuator design shown in FIG. 1. As shown in FIG. 1, a gimbaled electrostatic-gap actuator 100 consists of a base 102, an outer frame 104, and an inner part 108. The outer frame 104 is attached to a base 102 by a first pair of torsional flexures 106. The inner part 108 is attached to the outer frame 104 by a second pair of torsional flexures 110 positioned at a perpendicular angle relative to the first pair of torsional flexures 106. The gimbaled electrostatic-gap actuator 100 is suspended over a set of electrodes 112, and the angle of the gimbaled electrostatic-gap actuator 100 is adjusted by applying a voltage difference between the electrodes 112 and the suspended actuator 100. The gimbaled electrostatic-gap actuator requires a high voltage to achieve a significant angular deflection. Furthermore, the angle versus applied voltage characteristics of this structure are very nonlinear because the gap between the electrodes 112 and the suspended actuator 100 changes as the angle of the actuator is varied, and the electrostatic force between the electrodes 112 and the suspended actuator 100 has a nonlinear dependence on the gap. In fact, gap-closing actuators with linear restoring springs typically have a xe2x80x98snap-inxe2x80x99 instability point at approximately one-third of the full range of motion of the actuator. In addition, the two perpendicular axes of rotation can not be independently controlled since adjusting one axis changes the electrostatic gap associated with the other. This cross-axis dependence makes control of such structures difficult.
In the electrostatic gap-actuator structure of the prior art, the capacitance between the electrodes 112 and the suspended actuator 100 can be measured in order to monitor the position of the actuator. However, since the gap between the electrodes 112 and the suspended actuator 100 must be fairly large in order to allow for a large angular deflection, this capacitance is very small, and the accuracy of the capacitive measurement is very poor.
Vertical electrostatic comb-drive actuators have been employed to make one-dimensional rotational scanners. Electrostatic comb-drive actuators allow for exertion of a greater force over a large range by increasing the effective overall capacitive gap area. Furthermore, they allow for a more linear angle versus applied voltage relationship since the capacitive overlap area between the opposing electrodes depends almost linearly on the angle of the actuator, and the gap between the opposing electrodes remains fairly constant over the entire actuation range. Vertical electrostatic comb-drive actuators, which are shown in FIGS. 2A-B, have been used to produce one dimensional rotating mirror structures with significantly lower actuation voltages than required for electrostatic gap actuators as described in FIG. 1. FIG. 2A is a plan view of one-dimensional vertical comb-drive actuator 200 without applied voltage. The device 200 includes a base 202 and a mirror 204 attached to the base 202 by a pair of torsional flexures 206. Two vertical electrostatical comb-drive actuators containing movable comb fingers 208 and fixed comb fingers 210 are fixed to the base 202 and the mirror 204 such that the degree of engagement, or the overlap area, between the interdigitated comb fingers depends on the angle between the base 202 and the mirror 204. FIG. 2B is a plan view of the device illustrated in FIG. 2A with an applied voltage. As shown in FIG. 2B, applying a voltage to the actuators attracts the moving comb fingers 208 to the fixed comb fingers 210, which exert a torque on the mirror 204 and cause the mirror 204 to rotate about an axis 212.
The capacitive coupling between the moving comb fingers 208 and the stationary comb fingers 210 can be measured in order to monitor the angle of the mirror 204. Since the capacitance is fairly large, known methods can be employed to measure the capacitance with a high degree of accuracy. Similarly, comb-drives can be used for capacitive sensing only in a one-dimensional rotational actuator that employs another method of actuation (i.e. electrostatic gap-closing, magnetic).
U.S. Pat. No. 5,648,618 issued Jul. 15, 1997 to Neukermans et al., discloses a micromachined gimbaled actuator. An outer silicon frame oscillates around a first pair of bar shaped hinges by electrostatic or magnetic force. One end of each hinge of the first pair of hinges attaches to an inner frame, which attaches to a fixed inner post by a second pair of bar shaped torsion hinges positioned at right angles to the first set of hinges. The first and second pairs of bar shaped torsion hinges are made of single crystal silicon. First and second four-point piezoresistive strain sensors are built in the first and second pair of hinges for measuring the torsion displacement of the hinges. This apparatus does not posses several of the advantages gained by using comb-drive actuators and sensors, including linear behavior, low-voltage operation, and integration of the actuator and sensor in one structure. Furthermore, previous gimbaled structures have only employed lateral comb-drive actuators for in-plane motion.
There is a need, therefore, for an improved two-dimensional gimbaled scanner with out-of-plane rotational motion that provides linear drive and sense capabilities, low-voltage operation, and potential integration of the drive and sense mechanisms.
Accordingly, it is a primary object of the present invention to provide a gimbaled two-dimensional scanner, which contains vertical comb-drives that are used for actuation, for sensing, or for both actuation and sensing.
It is a further object of the present invention to provide a gimbaled two-dimensional scanner with vertical comb-drive actuators, which has two independently controlled axes of rotation.
These objects and advantages are attained by two-dimensional scanners containing vertical comb-drive actuators.
A two-dimensional scanner according to a preferred embodiment of the present invention consists of a rotatable gimbal structure containing a base, an outer frame, and an inner part. The outer frame is attached to the base by a first pair of torsional flexures that allow the outer frame to rotate about a first axis. The inner part is attached to the outer frame by a second pair of torsional flexures that allow the inner part rotate about a second axis. The inner part may include a reflective surface such as a mirror. The scanner further includes one or more vertical electrostatic comb-drive actuators positioned between the outer frame and the base, and between the inner part and the outer frame. Voltages are applied to the comb-drive actuators to allow the inner part to rotate about two axes. The voltages across the two pairs of comb-drives are adjusted to control the angle between the outer frame and the base, and angle between the inner part and the outer frame. Since the capacitance of the comb-drives depends on the degree to which they are engaged, the capacitance is measured to sense the angular positions of the inner part and the outer frame. The capacitive angle signals are used in servo loops to actively control the positions of the inner part and the outer frame.
Two-dimensional scanners having features in common with the two-dimensional scanner as described in above embodiment are used to produce fiber-optic switches that switch light between optical fibers.